Numerous studies of the effects of CNS stimulants on the behavior and cognition of boys with attention deficit-hyperactivity disorder (ADHD) have documented the short-term efficacy of these drugs on a broad range of dependent measures that include social behavior and cognitive performance, both measured in a variety of ways and settings. However, the overwhelming percentage of subjects in these studies have been ll years of age or younger. Given that it is clear that the majority of ADHD children do not remit at age 11, an important question for some time has been whether stimulant medication continues to be an efficacious treatment with ADHD children beyond the age of 11. Unfortunately, empirical data that address this question are virtually nonexistent. Therefore, the purpose of this investigation is to extend to a young adolescent sample the research that has been conducted with young ADHD children. A dose-response analysis will be conducted of the effects of methylphenidate on the cognitive and social behaviors of sixty 12 to 15 year old ADHD adolescents. In each of three consecutive summers, twenty 12 to 15 year old ADHD adolescents participating in a summer day treatment program will undergo a double-blind, placebo-controlled trial in which each adolescent will receive, for six days each with order randomized over days, no medication, placebo, .15, .3 and .6 mg/kg methylphenidate (MPH) b.i.d. Classroom dependent measures will include the following: accuracy and productivity on in-class assignments, attention to lecture, quantity and quality of notes taken, test performance, rule violations, and teacher ratings. Other dependent measures will be gathered in a variety of settings during the day treatment program, including recreational periods and group discussions, and will include frequency counts of positive and negative verbalizations directed towards peers and staff members, frequency counts of conduct problems and prosocial behaviors, rule violations, noncompliance to staff requests and commands, staff and parent ratings, laboratory measures of cognitive performance, and parent-adolescent interactions. Data analyses will address the following: (1) What is the shape of the function that relates dose to response for methylphenidate in this sample? (2) Does that shape vary across and/or within domains of response (e.g., cognitive versus social behavior; complex versus simple cognitive tasks; prosocial versus antisocial behavior)? (3) Are the obtained dose-response functions related to subject characteristics that include comorbid diagnosis of conduct disorder? Following their medication trials in the summer program, adolescents will be assigned to their best dose of medication given on either a b.i.d.-weekdays or t.i.d.-7 days schedule and followed for the subsequent school year to determine whether the effects of an acute trial of methylphenidate maintain with continued administration.